feat(chat): give MCP and tool rows their status, detail and a live toggle

[ElbertePlinio]

Frontend half of #362 and #365. Behind the new default-off mcpToolDetail flag.

Stacked on #387, which is what makes these events exist. Retarget to main once that merges — and note stacked PRs get no CI in this repo, so this needs a rebase-and-push after #387 lands to earn its checks.

The wire always carried this; the store threw it away

lib/agentChat.ts declares status and detail on both mcpToolCall and toolUse. The timeline item type did not, so the reducer read them and discarded them, and ChatTimeline rendered:

<McpCard server={item.server} tool={item.tool} />

With detail undefined, canExpand() is false → spacer instead of a chevron, disabled button, no body. That is exactly the reported symptom: MCP pickforge-lanes / lanes_wait as one dead grey line.

What changed

• Both item types carry status and detail, both optional — a row persisted before the parser learned to resolve non-Bash tools still renders rather than breaking on replay.
• A completion carrying no result keeps the arg summary already on the row. Resolving must never blank what the row was showing; that has its own test.
• Expansion moves into the hoisted RowExpansion map, the same contract CommandCard and ThinkingBubble use, so the toggle survives the virtualizer disposing the row on scroll-back. Chat transcript rows expand with no motion: only THINKING animates, CMD/TOOL/MCP/swarm/lanes snap open #372 listed that as a separate annoyance worth its own issue — it comes free here, because these cards had to take open/onToggle anyway.
• Status renders through StatusPill, matching CommandCard rather than adding new chrome: bracket-cornered mono, never a filled chip.

Why the flag

A persisted row's shape changed and the parser only emits completions on newer builds, so main stays releasable while this is dark — same precedent as changesReview gating the receipt card. flags.test.ts enumerates the registry, so the new flag is asserted there rather than silently appearing.

Tests

Three new store tests, all verified load-bearing (removing the carry fails two of them):

• an MCP call's status/detail carried, and the completion resolving in place rather than appending
• a completion with detail: null keeping the existing arg summary
• a generic tool reaching a terminal failed status

Validation

• bun run test:unit — 1604 passed
• bunx tsc --noEmit clean; bun run lint clean

The two new render branches are extracted out of renderItem because adding them inline pushed it past the repo's complexity cap.

Still not done for these two issues

Stated rather than implied:

• tool_progress elapsed time and system/status compacting/requesting (A running turn is opaque: WORKING names nothing, no elapsed time, generic tool rows show no args and never complete #365 B1) — needs a new event variant in event.rs. Without it, WorkingRow still says nothing about what is running, which is A running turn is opaque: WORKING names nothing, no elapsed time, generic tool rows show no args and never complete #365's headline complaint.
• WorkingRow naming the active tool — depends on the above.
• MCP tool rows in chat are dead ends: no args, no result, no completion — and pickforge-lanes should show lane cards #362 PR 3, lanes rendering inline as lane cards.
• Row-height estimates are unchanged: these rows gain a status pill but stay one line, so COMPACT_LINE_PX still holds. Worth re-measuring once elapsed time lands.
• No VRT fixture for MCP/tool rows with detail; fix(chat): space a run of tool rows as one block, prose as a new thought #383 added the row kinds but not a populated-detail scenario.

Refs #362, #365